The present invention relates to a method of packaging products and for cooling the packaged products. The method is particularly useful in the packaging and cooling of horticultural products and in field packaging of such products.
Several methods are commonly used for cooling perishable products where rapid cooling is required. These include hydrocooling, vacuum cooling, icing and forced air refrigeration. For example, the so-called "Desert Water Bag" operates on the principle that the evaporation of water from fabric forming the bag cools the water in the bag.
In the produce field, it is common to pick heads of lettuce and place them in waxed boxes with the box of lettuce then being hosed down with water either before or after the boxes are loaded onto a truck. Although evaporation of water from the lettuce during transportation assists in cooling the lettuce, relatively insignificant amounts of water are absorbed by the waxed boxes and cooling is limited. Transportation of broccoli in waxed boxes filled with ice is also known.
In addition, vacuum cooling approaches have been used for cooling produce. In accordance with this cooling technique, the warm product is loaded into an air tight chamber or tube which is subsequently evacuated by a mechanical or steam-ejector vacuum pump to establish a partial vacuum therein. As the total gas pressure in the tube is reduced below the saturation pressure of water at the temperature of the warm product (the "flash point"), water on and within the product begins to evaporate rapidly. The thermal energy required to provide the heat of vaporization of this water comes predominately from the sensible heat (e.g. "field heat") of the product. As a result, the product temperature begins to fall as rapid evaporation begins. Because vacuum pumps are generally very inefficient movers of condensable gases, such as water vapor, chilled coils are provided within the tube or chamber to condense and thereby remove the liberated water vapor. These coils are chilled usually by evaporation of liquid ammonia within, the ammonia being supplied by a conventional vapor-compression refrigeration unit.
In the absence of air or any other restriction to water vapor movement from the product to the chilled coil, the temperature of the product will in time equilibrate with that of the coil (the coil temperature in fact being commonly used as a control variable in vacuum cooling operations). Under these circumstances, the rate of thermal equilibration is largely determined by product characteristics. In general, products high in readily evaporated moisture content, with high thermal conductivity and high evaporative surface-to-volume ratio, will cool more rapidly under vacuum than do other types of products. For example, lettuce and other leafy vegetables cool well under vacuum (high moisture content and high surface-to-volume ratio), while melons do not (low evaporation rate and low surface-to-volume ratio). In addition, strawberries have not been viewed as suitable for vacuum cooling because of damage to the surface of the berries under vacuum conditions and the relatively small rise in cooling rate resulting from the vacuum conditions as opposed to nonvacuum refrigeration type cooling.
One example of a prior art vacuum cooling system is described in U.S. Pat. No. 4,576,014 to Miller, et al. In these approaches, water has been known to be added to the produce by sprinkling the produce before or while the vacuum is imposed to reduce the amount of moisture removed from the produce during cooling with the water evaporated during cooling being supplied at least in part by the water added to the system instead of entirely by the produce. In these approaches known to the inventor, the vacuum cooled produce sprinkled with water has been packed in waxed boxes which absorb very small amounts of water. All of these methods are significantly inhibited if product "exposure" is restricted, as when the product is packed in a plastic bag; such is the case where modified atmosphere packaging is used.
Modified or controlled-atmosphere packaging of fresh produce has also been heretofore utilized and offers advantages to virtually all sectors of the industry, from grower-shipper to food service and retail consumers. Benefits include reduced waste due to spoilage, enhanced quality, extended shelf life and greater consumer convenience. The essential feature of the modified-atmosphere approach to packaging is to seal the product in a package that restricts, to a predetermined degree, the exchange of gases between the product and the surroundings. Many studies have been performed on the desired gas environments for various types of products.
In general, modified-atmosphere packaging retards the four major causes of produce quality loss, namely dehydration, respiration, microbial spoilage and enzyme attack. The quality of cut fruits or vegetables (e.g. florets) deteriorates much more rapidly due to these factors than if the products remain uncut. Moisture loss from produce is governed by Fick's law of diffusion which states that the rate of vapor loss increases in direct proportion to the vapor pressure difference between the surface of the produce and the surrounding air. Since at a constant relative humidity, vapor pressure in the air nearly doubles for each 10.degree. C. temperature rise, and vapor pressure at the surface of fresh produce is nearly 100 percent, produce will dehydrate nearly four times faster at room temperature than at a temperature near freezing, when exposed to "dry" air. A modified-atmosphere packaging with a low moisture permeability will prevent this loss.
All produce continues to respire after harvest. During normal respiration, internal carbohydrates are converted into carbon dioxide, water and energy (heat) according to:
(aerobic respiration): C.sub.6 H.sub.12 O.sub.6 +6O.sub.2 .fwdarw.6CO.sub.2 +6H.sub.2 O+(heat).
This process generally results in a progressive deterioration in product quality. If a harvested item is stored in an oxygen depleted environment, anaerobic respiration occurs. This latter type of respiration is essentially a fermentation process that results in the production of an assortment of organic compounds that lead to undesirable flavors and odors. Anaerobic respiration is described as follows:
(anaerobic respiration): C.sub.6 H.sub.12 O.sub.6 .fwdarw.Alcohols+Acids +CO.sub.2 +H.sub.2 O+(heat).
Aerobic respiration rates can vary greatly among commodities, among varieties and even among parts of the same plant. There can be further variability due to growing conditions and post-harvest injuries, such as knife cuts, bruises, chill damage, etc. The most significant factors effecting respiration rate are the stage of maturity of the produce, temperature and storage atmosphere.
The "law of mass action" in chemistry states that the rate of a chemical reaction is proportional to the concentration of each of the reactants. Thus, aerobic respiration can be slowed by either decreasing the oxygen level or increasing the carbon dioxide level of the storage atmosphere. In practice, this relationship appears to hold with the result that increasing the CO.sub.2 level is equally as effective as decreasing the O.sub.2 level and that the results are additive. Plant sensitivity to CO.sub.2 ranges from low tolerance, as with apples, to high tolerance, as with strawberries.
Enzymes are organic catalysts present in abundance in produce. After harvest, these enzymes tend to "spill" from damaged, cut, bruised, etc. cells of produce and can lead to rapid discolorization of light colored surfaces, such as of mushrooms and cut apples. There are two basic ways to combat this enzyme activity. The first is through the reduction of the oxygen level in a package. Enzymatic browning rate tends to vary nearly linearly with oxygen concentration. The second approach is to use enzyme inhibitors. These are components that deactivate the browning enzyme. Sulfite, citric acid and ascorbic acid additives have been used for this purpose. In addition, carbon monoxide in concentrations of one to ten percent is effective as an enzyme inhibiter and as a microbicide. Items known to benefit from small (one to five percent) concentrations of carbon monoxide include cauliflower, avocados, strawberries, tomatoes, cherries and grapes. Items known to benefit from larger concentrations (five to ten percent) include lettuce, stone fruit, melons, cantaloupe, mushrooms and citrus products.
Although bacterial diseases can cause significant decay in vegetables, most post-harvest diseases are caused by fungi. Since these organisms respire in the same manner as the cut plant, their growth in general is controlled by the same factors (e.g. high CO.sub.2 concentration, etc.). In addition, microbial decay is dramatically accelerated under high relative humidity conditions. There are a variety of chemical treatments used to control these pathogens, including carbon monoxide and sulfur dioxide. Related to controlling microbial decay of produce, is the control of insects, in particular with respect to exported products which are frequently subjected to quarantine fumigant treatments.
It is also known to inject or charge modified-atmosphere containers with gas of a desired composition for the particular products. This approach has been used, for example, in connection with bread whereby bread is placed in plastic wrappers which are injected with gas of the desired environment prior to sealing the bread in the wrappers. In addition, poultry products are packaged in high CO.sub.2 environments and red meat products are packaged in high O.sub.2 and CO.sub.2 environments.
Because modified-atmosphere packaging inhibits the action of these major causes of product quality loss, it has recently been a focus of much activity. In this regard, there is much data which describes the optimal atmosphere for a variety of commodities. For example, the article entitled "Post-Harvest Technology of Horticultural Crops", by Kader, A et al, special publication 3311, published by the University of California at Davis in 1985, contains a table of optimal storage atmospheres for a wide variety of types of produce. Controlled atmosphere packaging has also been used for bakery, meat and other perishable food products. In general, it appears that one can deviate substantially from an optimal atmosphere and still benefit. Modified-atmosphere packaging is also the subject of numerous patents, such as U.S. Pat. Nos. 4,256,770 to Rainy; 4,515,266 to Myers; and 4,910,032 to Antoon, Jr.
Although these technologies exist, when produce is enclosed in a modified-atmosphere package, it becomes difficult to remove heat, such as heat in the produce and existing at the harvest site or field. In addition to this trapped field heat, the produce continues to warm due to the heat of respiration. As temperature rises, respiration increases exponentially, resulting in heat build up. This situation can readily lead to a loss of product quality that quickly negates the benefits intended with the modified-atmosphere package.
In the prior art, due to the fact that controlled-atmosphere packaging involves the sealing of products in a package that restricts the exchange of gases between the product and surroundings, conventional techniques for field heat removal, such as forced-air cooling and hydrocooling have been applied before the product is sealed in its package and palletized. Because the equipment associated with the cooling techniques is usually located at a central location, the use of modified-atmosphere packaging systems generally requires that the product be shed-packed at a location remote from the picking location, in contradiction to recent trends in agriculture favoring field-packing of many fresh produce items. In addition, if the ready escape of water vapor from the product surface and/or its subsequent flow to a chilled condensing coil are restricted, the rate of cooling under vacuum may be significantly reduced, even in the case of otherwise readily-cooled items, such as lettuce. By their very nature as gas-flow regulating devices, typical modified-atmosphere packages would be expected to inhibit the vacuum cooling process, owing to the severely restricted rates of gas (water vapor) removal from the package.
Thus, the standard modified atmosphere approach for packing berries, such as strawberries, is to pick or harvest the berries into containers; palletize the containers of berries and refrigerate the pallets. After the berries are cooled, the pallets of berries are wrapped in plastic and injected with an enriched CO.sub.2 mixture and shipped. When the pallets reach the distributors or end users, the pallets are broken apart and the benefit of the modified atmosphere packaging is lost at that point.
For most modified atmosphere packaged produce other than berries, the produce is harvested and transported to a remote shed for cooling. The cooled produce is cut, processed and sorted. The cooled and now processed produce is then packaged in a modified atmosphere container. This approach is costly and results in damage to the produce due to multiple handling steps and due to the delayed placement of the produce in a modified atmosphere package.
Therefore, a need exists for a new method for overcoming these and other disadvantages of the prior art.